In four-high rolling stands for strip and sheet the state of the art includes the use of bending and possibly balancing blocks in cooperation with the chocks on which the rolls are mounted.
These bending and balancing blocks include jacks, or more generally compression units, which act on the two sides of the chocks to generate a greater or lesser thrust so as to bend the relative roll in a manner correlated, among other things, to the deflection to which the roll is subjected during the rolling process, to the mechanical rounding of the rolls, to their thermal expansion and their wear.
The jacks act alternately on the two faces, upper or lower, of the relative chocks so that, when driven, they can impart a positive bend ("IN" bend), if the bend contrasts the flection of the rolls generated by the rolling load, or a negative bend ("OUT" bend), if the bend acts to increase the flection of the rolls.
U.S. Pat. Nos. 4,773,246, 4,038,857, 4,976,128 and 3,307,386 show some examples of four-high rolling stands equipped with bending and balancing blocks.
The state of the art also covers the need to induce two different movements on the working rolls: a crossing movement on the rolling plane, which causes the rolls to assume a reciprocally crossed position, which may be performed both during the working step and during the initial set-up step of the stand, and also an axial movement, or shifting, of one roll with respect to the other.
In order to achieve this crossing of the rolls, a plurality of solutions have been proposed in the state of the art, for example including screws, gears, jacks, etc.
Moreover, solutions have been proposed which include the use of cam or eccentric elements laterally coupled with the outer face of the chock and suitable to cause it to be laterally displaced in response to a rotation imparted thereto in order to cross over the rolls.
The cam or eccentric elements do not normally act directly on the outer lateral face of the chock, but on intermediate elements, such as gibs, plates or similar.
Some embodiments have been proposed where the intermediate elements are E-shaped or F-shaped so that the bending jacks which act on the relative chocks are housed on the fins defined by this shape.
Those embodiments known to the art where there is both bending and crossing of the rolls include either jacks mounted on extensions made directly on the housing of the stand (U.S. Pat. No. 3,307,386) or jacks mounted on the supporting fins or again jacks which, in order at least to induce the negative bend, are mounted on the chocks of the back-up rolls and act on the chock of the relative working roll.
In the first case, it is very complex and expensive to make the seatings for the jacks on the housings.
In the third case there is the problem that, when the rolls have to be dis-assembled and re-assembled with the relative chocks, it is necessary to disconnect and then reconnect the hydraulic system which feeds the jacks.
This disconnection, apart from causing a delay in the operations to change the rolls, causes a serious danger of infiltration of dirt into the connections, with consequent serious risks of damage to the servovalves and for the components of the oil-dynamic circuit.
Moreover, in this embodiment there is a need to use flexible cables to connect the jack, which is movable with the chock, to the relative feed circuit; these flexible cables cause operating difficulties in that they reduce the dynamic performance of the system which controls the pressure and force exerted by the jacks.
Another problem in the state of the art is that the ends of the pressure elements of the jacks slide on the fins of the chocks when the shifting and/or crossing movement is carried out in a condition of activated load.
A further problem is to achieve means to constrain/release the bending block, in a simple and rapid manner, either to/from the relative chock so that, during the working steps, the block follows the movements of axial translation imparted to the rolls, or to/from the stationary housing during the steps when the rolls are replaced and extracted.
Yet another problem is that the axes of the jacks, during the shifting movement, may be misaligned with respect to the centre line of the bearings of the working rolls; this causes a problem in that the load is not symmetrical and therefore there is a deterioration and premature wear of the bearings themselves.
EP-A-0 744 227, in the name of the applicant, discloses a device for the crossed displacement of rolling rolls, comprising motion transmission means and actuation means arranged at least on a side of a respective chock. It does not disclose means for performing positive or negative curvature of the rolls, i.e. the bending of the rolls.
EP-A-0 744 228, also in the name of the applicant, discloses a block to provide curvature of the rolling rolls in a four-high rolling stand, the block comprising lower and upper ribs including jack means cooperating with the opposite surfaces of the chocks and compensation jack means acting on the chocks of the corresponding back-up roll.
It does not solve the problems above mentioned, and in particular the problem of the sliding of the pressure elements of the jacks on the fins of the chocks when the shifting and/or crossing movements of the rolls are carried out, and of the misalignment of the axes of the jacks with respect to the center line of the bearings of the working rolls.
The present applicant has designed and tested this invention to overcome all these problems and to achieve further advantages as shown hereinafter.